When both aufs and overlayfs are available (e.g. Ubuntu 20), cvmfs picks aufs as a default. It's time to switch the default to overlayfs.
The hardlink treatment should be also adjusted in:
- cvmfs_server_import.sh and cvmfs_server_import.sh: The warning "Warning: CernVM-FS filesystems using overlayfs may not enforce hard link semantics during publishing" is no longer needed for overlayfs.
- sync_mediator.cc: We do not need to break hardlinks.
- cvmfs_server_migrate.sh: CVMFS_IGNORE_XDIR_HARDLINKS should be set to true by default.